Prekallikrein (PPK) and total kininogen (KGN) were measured in human plasma. PPK was determined by radiochemical esterolytic assay, KGN by radioimmunoassay as bradykinin after trypsin digestion of plasma. In normal volunteers, no changes in PPK and KGN were effected by variation of dietary potassium, administration of fludrocortisone, or dehydration. In hypertensives, no variation in PPK or KGN was observed with manipulation of dietary sodium, administration of indomethacin, or administration of SQ 20,881. In patients with systemic lupus erythematosis, aspirin therapy did not change PPK, but did effect a small decrease in KGN.